I. Field of the Invention:
This invention relates generally to a drive system for a marine craft, and more particularly to an improved surface drive for a marine craft incorporating an inboard engine for powering the craft.
II. Discussion of the Prior Art:
Various arrangements are known in the art for providing motive power to a boat. For smaller crafts, the most common source of motive power is the outboard motor. It generally comprises a portable, unitary structure including a power head incorporating an internal combustion engine having a drive shaft extending vertically downward within an exhaust gas housing and leading to a lower unit comprising a transmission gear mechanism for coupling the engine's vertical drive shaft to a horizontal propeller shaft. This unitary assembly is suspended from the boat's transom such that the propeller mounted on the propeller shaft is totally submerged to a depth which is below the boat's keel.
For boats of a somewhat larger size, one or more engines, each including anywhere from four to eight cylinders, will be physically located within the boat's hull. The engines' shafts extends through the transom to individual outdrive units. Such outdrive units will each comprise a vertical shaft having a pinion gear on opposed ends thereof, the upper pinion gear cooperating with a bevel gear driven by the engine shaft and the lower pinion gear being arranged to mate with either a forward or a reverse bevel gear affixed to a horizontally disposed propeller shaft. The above-mentioned parts comprising the outdrive are generally contained within a casing which extends downward into the water such that the propeller will be totally submerged. Hydraulic cylinders are provided for pivoting the outdrive between a raised and lowered position to permit the attitude of the boat to be trimmed and to properly compensate for the passenger load. Moreover, means are provided for steering the craft by pivoting the outdrive about a vertical axis.
A third type of drive system is termed an "inboard" and here, both the engine(s) and the transmission(s) are contained within the hull and only the propeller shaft extends outward through the transom or through the bottom of the boat. With this type of drive, steering is generally accomplished by a separate rudder rather than by redirecting the orientation of the propeller shaft.
All three of the above conventional drive systems for marine craft exhibit a reduced efficiency due to "appendage drag". That is to say, these drives each involve an appendage, i.e., the lower unit portion of the outboard or outdrive which is immersed in the water and at a level beneath the craft's hull. Dragging this appendage through the water necessarily detracts from the forward speed of the craft.
A somewhat recent innovation in marine craft drive systems is the so-called surface drive described in the Arneson U.S. Pat. No. 4,544,362 and the Adams et al U.S. Pat. No. 3,933,116. In these arrangements, the propeller shaft is brought out through the transom or the outdrive is arranged such that it assumes a generally horizontal orientation just at the waterline such that only one-half of the propeller hub is submerged. The prop shaft and its housing are driven through a universal joint coupling the engine shaft to the propeller shaft and allowing the propeller shaft to be pivoted, as by a suitable hydraulic actuator, in both a horizontal and a vertical plane. This arrangement cuts down significantly on the appendage drag in that approximately only one-half of the prop shaft extension housing is submerged and it will generally be projecting rearward from the transom at a level above the bottom or keel of the craft.
The Arneson patent also discloses the use of a constant velocity universal joint for rotationally coupling the engine's or transmission's shaft to the propeller shaft and to the prop shaft extension housing. Such a constant velocity U-joint generally comprises an outer race having spherical grooves formed in its inner surface and an inner race in the form of a ball or sphere also having spherical grooves formed in its exterior surface. A plurality of drive balls are disposed in an annular cage and have a radius corresponding to the radius of the spherical grooves formed in the inner and outer races so that the drive balls can fit within the respective grooves. The engine shaft or transmission shaft is coupled to the inner race so as to impart rotational forces to it. These forces are then transmitted through the drive balls to the inner race. The outer race is, in turn, coupled to the propeller shaft. By providing this mechanism, the rotational forces can be applied to the propeller at the same time that the prop shaft can be pivoted vertically and laterally relative to the engine shaft.
With the constant velocity U-joint shown in the Arneson patent, all of the forward thrust forces developed by the propeller must be transferred through a spherical housing surrounding the outer race of the constant velocity U-joint and from there through a tubular support casing directly to the boat's transom. In that these thrust forces are quite substantial, it dictates that these structures be sufficiently large to surround the U-joint and strong enough to carry the prop thrust forces. This adds to the appendage drag presented by the drive assembly.
As is further set out in the Arneson patent, in a surface drive it is desirable that the ball socket surrounding the U-joint be disposed as low as possible on the boat transom with the propeller shaft maintained in close longitudinal alignment with the drive shaft during normal forward travel of the boat. This causes the line of propeller thrust to be maintained low relative to the boat and below the boat's center of gravity. The ability to position the tubular ball socket low on the transom is dependent upon the size of the ball socket itself and, hence, if a U-joint of lesser dimension can be employed, the size of the ball socket surrounding the U-joint can be reduced.
Another feature of the Arneson drive is that the exhaust manifolds and exhaust pipes for the engine are located above the water line and pass through the boat's transom. As such, engine exhaust noise tends to be excessive. The noise can be reduced if the exhaust gases and engine cooling water are allowed to exit through the propeller extension housing.